Probably the only actual human beings whose utility functions (in so far as they have them) are perfectly selfish are sociopaths, or very odd psychologically in other ways. But considering hypothetical agents with perfectly selfish utility functions is mostly just a convenient approximation. (And it’s not that bad an approximation; most people are very well approximated, in many of their interactions, as perfectly selfish agents, and the approximation errs more badly by assuming that they have utility functions than by assuming that they’re perfectly selfish. I think.)
Probably the only actual human beings whose utility functions (in so far as they have them) are perfectly selfish are sociopaths, or very odd psychologically in other ways. But considering hypothetical agents with perfectly selfish utility functions is mostly just a convenient approximation. (And it’s not that bad an approximation; most people are very well approximated, in many of their interactions, as perfectly selfish agents, and the approximation errs more badly by assuming that they have utility functions than by assuming that they’re perfectly selfish. I think.)